Very Large Array Limits For Intermediate-Mass Black Holes In Three Globular Clusters
The observational evidence for central black holes in globular clusters has been argued extensively, and their existence has important consequences for both the formation and evolution of the cluster. Most of the evidence comes from dynamical arguments, but the interpretation is difficult, given the short relaxation times and old ages of the clusters. One of the most robust signatures for the existence of a BH is radio and/or X-ray emission. We observed three globular clusters: NGC6093 (M80), NGC6266 (M62), and NGC7078 (M15), with the Very Large Array (VLA) in the A and C configuration with a 3-sigma noise of 36, 36, and 25 mu Jy, respectively. We find no statistically significant evidence for radio emission from the central region for any of the three clusters. NGC6266 shows a 2-sigma detection. It is difficult to infer a mass from these upper limits clue to uncertainty about the central gas density, accretion rate, and accretion model.